Throughout maritime history navigators have sought an accurate, reliable method of determining their position on the surface of the earth and many instruments such as the sextant were devised. During the second World War, a long range radio navigation system, LORAN-A, was developed and implemented under the auspices of the United States Coast Guard to fulfill wartime operational needs. At the end of the war there were seventy LORAN-A transmitting stations in existence and all commercial ships, having been equipped with LORAN-A receivers for wartime service, continued to use this navigational system. This navigational system served its purpose but shortcomings therein were overcome by a new navigational system called LORAN-C.
Presently, there are eight LORAN-C multi-station transmitting chains in operation. This new navigational system will result in an eventual phase-out of the earlier LORAN-A navigational system.
LORAN-C is a pulsed, low-frequency (100 kilohertz), hyperbolic radio navigation system. LORAN-C radio navigation systems employ three or more synchronized ground stations that each transmit radio pulse chains having, at their respective start of transmissions, a fixed time relation to each other. The first station to transmit is referred to as the master station while the other stations are referred to as the secondary stations. The pulse chains are radiated to receiving equipment that is generally located on aircraft or ships whose positions are to be accurately determined. The pulse chains transmitted by each of the master and secondary stations is a series of pulses wherein each pulse has an exact envelope shape, each pulse chain is transmitted at a constant precise repetition rate, and each pulse is separated in time from a subsequent pulse by a precise fixed time interval. In addition, the secondary station pulse chain transmissions are delayed a sufficient amount of time after the master station pulse train transmissions to assure that their time of arrival at receiving equipment anywhere within the operational area of the particular LORAN-C system will follow receipt of the pulse chain from the master station.
Since the series of pulses transmitted by the master and secondary stations is in the form of pulses of electromagnetic energy which are propagated at a constant velocity, the difference in time of arrival of pulses from a master and a secondary station represents the difference in the length of the transmission paths from these stations to the LORAN-C receiving equipment.
The focus of all points on a LORAN-C chart representing a constant difference in distance from a master and a secondary station, and indicated by a fixed time difference of arrival of their 100 kilohertz carrier pulse chains, describes a hyperbola. The LORAN-C navigation system makes it possible for a navigator to exploit this hyperbolic relationship and precisely determine his position using a LORAN-C chart. By using a moderately low frequency such as 100 kilohertz, which is characterized by low attentuation, and by measuring the time difference between the reception of the signals from master and secondary stations, the modern day LORAN-C system provides equipment position location accurate within two hundred feet and with a repeatability of within fifty feet.
The theory and operation of the LORAN-C radio navigation system is described in greater detail in an article by W. P. Frantz, W. Dean, and R. L. Frank entitled "A Precision Multi-Purpose Radio Navigation System," 1957 I.R.E. Convention Record, Part 8, page 79. The theory and operation of the LORAN-C radio navigation system is also described in a pamphlet put out by the Department of Transportation, United States Coast Guard, Number CG-462, dated August, 1974, and entitled "LORAN-C User Handbook."
The signals presently received by LORAN-C navigation receivers have very low signal-to-noise ratios and it is difficult to locate the third cycle positive zero crossing conventionally used in making the time difference measurements between signals received from the master and secondary stations.
Automatic LORAN-C receivers presently acquire or locate signals being received from the master and secondary stations of a selected LORAN-C transmitter chain and then go into a tracking mode wherein they calculate the time of arrival of future received signals. More particularly, the receiver calculates the time interval between the receipt of the master station signal and each of the associated secondary stations and then within relatively narrow time windows it looks for the signals from each of the secondary stations. As a result of noise, the measured time of reception of each secondary station signal varies within the time window in which it is received and may be represented by a bell shaped distribution curve. The peak part of the distribution curve represents the true time of arrival of the secondary station signal. As the signal-to-noise ratio decreases, the distribution curve representing signals received over some finite period of time becomes broader and the central peak is less pronounced in a manner well understood in the art.
On the LORAN-C receiver the position display is affected by received noise. In a very good signal-to-noise environment the position display is stable and provides accurate navigation information. However, as the signal-to-noise ratio of the received signals decreases, even to those levels normally encountered in operation, the least significant digit on the LORAN-C receiver display begins "jump around" and cannot be relied upon. This decreases the accuracy of the navigation information provided via the display.
To overcome this problem prior art LORAN-C receivers set the time constant of their tracking servos to in the order of twenty seconds. This is a compromise figure which normally provides acceptable operation on relatively slow speed craft such as boats, but as the signal-to-noise ratio decreases further, the navigation information output again becomes unreliable. The time constant of the receiver tracking servos cannot be increased further or the receiver will lose track of the master and secondary stations fairly easily and must reacquire or relocate them. In addition, such prior art LORAN-C receivers having tracking servo time constants of twenty seconds cannot be used on high speed craft such as airplanes for they easily lose track of the LORAN-C transmitter signals. Having a relatively long servo time constant such as twenty seconds also means that when the receiver is turned on it takes twenty seconds or more to provide navigation information to the receiver operator.
In a good operating environment a LORAN-C receiver can tell the position of the receiver within about fifty feet, but with the unreliability of the least significant display digit in poor operating environments the level of accuracy is reduced to about twenty-five hundred feet. This increased inaccuracy has long been a problem in the prior art when signal-to-noise ratio is below a certain level.
The foregoing problem in the prior art, is satisfied by my novel method for making corrections to the measured time delays between receipt of master and secondary station signals to remove the effects of noise before the measured time delays are integrated and used to provide navigation information. The effect is to decrease the signal of the bell-shaped distribution curve of measured time delays over a finite period. The end result is to shorten the time constant in the servo tracking loop in the LORAN-C receiver while at the same time increasing reliability of the displayed navigation information in significantly lower signal-to-noise ratio operating environments than has heretofore been possible in the art. In addition, the receiver utilizing my novel method may also be used on higher speed craft.
The present invention will be better understood upon a review of the description given hereinafter in conjunction with the drawings in which: